Data Usage- Does it include WiFi data?

I ask because i've been at home on wi-fi all day today, and watched a youtube video. When I checked my data usage via settings, it said youtube has used 40MB of data. It was my assumption that the data usage tool only tracked mobile data- is this not the case? I'm looking back at previous days and it's saying my podcast app has been doing something similar, even though I make a conscious effort to only download podcasts when at home on wifi.